K is smaller than H, but K is the heaviest. H is largest because of volume, which is inferred from neck thickness.

None of this makes sense.

You can't derive animal volume from neck thickness. Until figures are given for volume, size has to be measured as mass (which, on Earth, is similar to weight). K is the largest, not H. (If K and H have the same density, K has both greater weight and volume.)

As for the photo...it isn't the large dog, it's its (couldn't resist writing that) large head that casts doubt.

OKAY DOG PEOPLE! HERE'S ONE FOR YOU! I THINK THEY USED HIM AS THE MODEL FOR THE 3 HEADED DOG IN ONE OF THE HARRY POTTER MOVIES!

Just had to share this. Check the picture!

Hercules: The World's Biggest Dog Ever According to Guinness World Records
Hercules was recently awarded the honorable distinction of Worlds Biggest
Dog by Guinness World Records. Hercules is an English Mastiff and who has a
38 inch neck and weighs 282 pounds.

With "paws the size of softballs" (reports the Boston Herald), the
three-year-old monster is far larger and heavier than his breed's standard
200lb. limit. Hercules owner Mr. Flynn says that Hercules weight is natural
and not induced by a bizarre diet: "I fed him normal food and he just
"grew"... and grew. and grew. and grew.

Kell: Slightly Smaller, But the World's Heaviest Dog

Hercules' sheer volume may have won him the Largest Dog world record, but
the Heaviest Dog title still rests with Kell who weighed in at 286 pounds in
August of 1999. This English Mastiff, however, only has a 32-inch neck - far
less than Hercules' 38-incher. Think about that for a second though - 32 inches
is a standard waist size for a man!

Proud owner Tom Scott said Kell is two years old and is expected to
continue to grow for the next two years. Kell needs to be fed 100lb of beef
every week, and drinks gallons of goats' milk to stay healthy...

They don't know their breeds or their world records terribly well.

As has already been stated, the dog in the OP is clearly a neo mastiff. Here's an English Mastiff (yeah, I know... wikipedia). The wiki article also has info on the Guiness BoWR entry for the largest of the breed who is neither Hercules or Kell.

Going a bit OT here, but this thread makes me wonder about the extra-cost "largest horse" or "largest steer" attractions you see at county fairs. I've never felt like paying for admission and I wonder if there's a real horse in there or if there's some alteration going on.

With that particular image, it looks to be about the size of an Irish Wolfhound, provided those two people are of normal height. That's still way too big for an average Neo, but they could just be short people.